1. Field of the Invention
The present invention generally relates to an ultrasonic imaging apparatus for acquiring information of a medium by utilizing ultrasonic beams, and more particularly, to an apparatus for measuring the ultrasonic velocity in a local region of interest within the medium by means of crossed ultrasonic beams.
2. Description of the Related Art
Diagnostic methods using ultrasonic beams can provide a proper diagnosis of a soft tissue without harmfull effects on a biological object under examination, i.e., with noninvasive effects. In this respect, therefore, such diagnostic methods recently become popular with an improvement of high speed ultrasonic scanning apparatus. Another reason for the popularity is that diagnostic methods involve a pulse echo technique, which has a higher operability and is less limited with respect to diagnostic regions of interest (referred to as a "ROI") as compared with the transmission technique.
Recently, there is a growing demand of differential diagnostic method. This differential diagnosis is considered to be particularly effective in diagnosing the malignancy of an pathological organ or organic tissue. However, according to a conventional pulse echo technique, the intensities of echoes depend on the shape of a reflection surface, the incident angle of ultrasonic beams to the reflection surface and the amount of ultrasonic beams absorbed within a biological body, as well as a variation in the acoustic impedance (product of tissue density and ultrasonic velocity) of internal tissue structures. What is more, these intensity data are difficult to separate for detection. Naturally, the differential diagnosis for the ultrasonic diagnostic methods is considerably difficult.
On the other hand, a method of measuring only the ultrasonic velocity is executed in a transmission type ultrasonic CT (Computer Tomography). Although this method is still under development, the measuring accuracy is gradually being improved (Publication: Greenleaf, J. F. et al. Acoustical Holography Vol. 6 1975). However, since the transmission technique cannot directly apply to measure velocities in a region which includes a bone or gas in its ultrasonic propagation path, it is therefore very disadvantageous in that this technique may be applicable only to a limited field, such as a mammary-gland diagnosis.
Furthermore, very recently, the pulse echo method is modified to measure the ultrasonic velocity within an internal organ of a biological body. FIG. 1 shows an example of the method of measuring the ultrasonic velocity in a liver reported by Nishimura et al. (Proc. 44th Mtg Jpn Soc Ultrason Med 129-130 (1984). Akamatsu et al. method employs two ultrasonic transducers 151 and 152, having a strong directivity (the directions indicated by the one-dot and dash line), as a transmitter and a receiver, respectively. The transit time of the ultrasonic beams travelling from transmitting transducer 151 to receiving transducer 152, reflecting at the proximity of the intersection O of the central axes PP' and QQ' of the respective transducers 151 and 152, is measured. The average ultrasonic velocity in the liver is derived from this transit time and a predictive propagation distance (i.e., distance POS), which is calculated by the distance .DELTA.X and incident angle .theta. between transducers 151 and 152.
Since the ultrasonic velocity obtained by this method is equal to the average ultrasonic velocity in the propagation path, this method is effective in diagnosis of diffused liver disease such as hepatocirrhosis which has a uniform acoustic characteristic over the entire liver. However, it cannot apply to measure the ultrasonic velocity in a local affected region. Even this diagnosis should consider the ultrasonic velocity at the epidermis or layers of fat, and muscle whose velocity is different from the velocity in liver, thus increasing an error in the measurement of the ultrasonic velocity.
For instance, when the ultrasonic velocity, C.sub.0, over a living tissue is uniform in FIG. 1, the ultrasonic velocity can be accurately obtained from the equation: ##EQU1## where "t" is the measured propagation time. However, when the ultrasonic velocity of a surface layer (e.g., a layer of fat or muscle) 153 differs from the velocity in a diagnostic region 154, ultrasonic beams reflect at points "R" and "S" at which the ultrasonic beams from transmitter transducer 151 and those reaching receiver transducer 152 respectively intersect the boundary surface between diagnostic region 154 and surface layer 153. Therefore, the actual propagation distance (RO'S) differs from the pre-estimated propagation distance (ROS). Consequently, the ultrasonic velocity in diagnostic region 154 cannot be accurately obtained from equation (1).
According to the conventional ultrasonic velocity measuring apparatus, if the ultrasonic velocity of a surface layer structure differs from the velocity in a region of a medium ultrasonic beam refraction occurs at the boundary surface between the diagnostic region and the surface layer structure. This causes an error in the propagation distance of the beams so that the ultrasonic velocity in the region cannot be accurately estimated.
It is therefore an object of this invention to provide an ultrasonic local velocity measuring apparatus, which is hardly influenced by the surface tissue layer having the different sound velocity from the velocity in a region of interest in a medium.